Method of testing oils

ABSTRACT

A method of testing oil for unstable reactive compounds include reacting unstable compound from an oil sample with an acid catalyst to form a reaction product, the color of which is then related to the presence and/or amount of reactive compound in the oil. Kits combining the necessary materials and reagents for performing the test method are also provided.

This invention relates to a method of testing oils for potentialinstability and to test kits for applying the test method.

Unstable oils undergo chemical degradation reactions which, over periodsof time, produce small but insignificant changes in their properties.The amount of insoluble matter and/or sediment in the oil is one suchproperty. These materials can plug the fuel channels in fuel filters andreduce or alter the spray pattern from engine injector nozzles. In bothcases the required flow and delivery of fuel to the engine combustionchamber cannot be maintained and poor engine performance and possiblemechanical damage may result.

Chemical reactions associated with unstable oils may also occur at anaccelerated rate when the oil is heated. For example, hot metalcomponents in fuel systems heat the fuel and may result in forminginsoluble matter. This matter can form an organic deposit or varnish onsuch surfaces. The deposits may reduce the rate of heat exchange throughthe metal surface or alter the flow of fuel from those of the designrequirements.

Concern with respect to the storage stability of oils is increasing asrelative demand for the possible range of refinery product from crudeoil has resulted in increased use of refinery cracking processes toconvert part of the heavier ends to lighter middle distillate fuels. Thecracked products, which contain higher amounts of chemically unstablespecies, are blended with more stable straight run streams to producecommercial fuels.

The strong influence of these unstable species in cracked refinerystreams on deposit formation from the fuel is recognised by the oilindustry, which frequently uses further refinery processing in the formof hydrogenation to reduce the concentration of these unstable species.Alternatively, or in addition, stabilizing additives may be added to thefuel which propose to reduce the extent of deposit formation.

The chemical reactions which produce insoluble materials in fuel areslow at ambient temperature and very little change can be detected evenin a very unstable fuel during the short storage time at the refinery.It is believed that one important mechanism by which sediments areformed is reaction between trace compounds present in the oils to formsolid or gummy products. Therefore even repeated filtration of the oilsmay not solve the problem of sediment formation. "Fuel" 67 (August 1988)1124-1130 and 68 (January 1989) 27-31 suggest one of the principalreactions involved in sediment formation in diesel oils is the reactionbetween phenalanones (I) or Phenalenones (also known asperinaphthenones) (II), which are themselves produced by oxidation ofphenalenes (also known as perinaphthalenes or benzonaphthenes) (III),##STR1## and indoles to form sediment precursor compounds of formula(IV) where R is alkyl and n is 1 to 3: ##STR2## In formula (I) to (IV),the tricyclic ring compounds may be substituted, for example by one ormore alkyl groups. Phenalenes, phenalanones, phenalenones and indolesare believer to be introduced in the course of modifying the oils byincorporation of catalytically cracked oils.

The 1989 article describes reactions between synthesised phenalanones orphenalenones and indoles under acid conditions to yield intensely blueindolyl phenalene salts.

A number of methods have been developed to increase the rate at whichdeposits are formed so that meaningful amounts may be measured. The mostcommon is the use of American Society for Testing and Materials (ASTM)Method D2274, which uses an elevated temperature of 95° C. for 16 hoursand the bubbling of oxygen gas through the fuel. A recent experimentaltest method is to heat the fuel at 90° C. in an oxygen pressure bombwith an oxygen gas pressure of 794 kPa for 16 hours. This method isknown as the oxygen overpressure test. Concern has arisen from thosepractising this art that the high temperatures and the presence ofoxygen result in different chemical sediment producing reactions thanoccur at ambient temperatures. Gravimetric measurements of the amount ofsediment produced under such conditions may not correlate with thatproduced on storage at ambient temperatures.

Much milder test conditions are found in the DEF STAN 05-50 (Part40/1--long method) in which fuel (700 ml) is aged in a 1000 ml glassbottle for 28 days at 50° C. The gravimetric amount of sediment producedin this test is considered to give an approximation to the amount ofsediment produced at 1 year of ambient storage and a good indication ofthe storage stability of a fuel. The long duration required to carry outthis test makes it impractical for commercial decision making withrespect to fuel stability prognostication.

The requirement for more rapid methods has lead to German (East) PatentDD 207580. Chromium (VI) is reduced by fuel in an aqueous medium and thechange in light transmission in the medium measured at 460 nanometers.The amount of reduction of chromium (VI) is reported to be proportionalto the oxidative stability of the fuel. The art of U.S. Pat. No.4,556,326 is the measurement of the light transmission of a sample offuel before and after it had been heated in the range 93° C. to 171° C.for from 3 minutes to 90 minutes.

It is the objective of this invention to provide a simple rapidalternative means to determine the relative stability of a fuel.

According to this invention there is provided a method of testing an oilfor unstable reactive compounds characterised by contacting at least onesaid reactive compound from a sample of the oil with an acid catalyst toform a coloured reaction product, then relating the visible colourand/or colorimetric absorbance between 600-850 nm of this product to thepresence and/or amount of unstable reactive compounds in the oil.

This method is applicable to oils which form sediments by reactions ofpolynuclear aromatic species and heteroaromatic species. The oils may benatural, e.g. petroleum oils or shale oils, or may be wholly or partlysynthetic, e.g. prepared from natural gas, from coal or by heatingshale, or in any other way. The term "oils" as used herein specificallyincludes petroleum spirit (gasolines), naphthas, paraffins (kerosine),distillate fuels such as tractor fuels, diesel fuels, gas oils and gasturbine fuels, lubricating oils, cutting oils and hydraulic oils. It isespecially applicable to those oils in which sedimentation is a problemsuch as diesel fuels, gas oils and gas turbine fuels. It has been foundto be applicable both to oils which are fresh from the refinery andwhich have been stored for a long time, for example several months.

The reactive compounds in the oil can be contacted with the acidcatalyst in any suitable manner. For example, a sample of the oil may beadded to the catalyst, or catalyst may be added to the oil sample.Various other materials which may be used to make the method of testingmore sensitive and/or easier to operate are described below. Thereactive compounds may be separated from the oil sample before they arecontacted with the catalyst.

Although it is not limited to any specific scientific theory, it isbelieved that the test relies upon the oxidation of phenalenes by theoxidising agent to phenalenones, and the subsequent formation of acoloured indolylphenalene salt in the presence of acid. Such salts aregenerally blue to blue-violet in colour, but the colour formed if thesepotentially sediment-forming compounds are present may vary between blueand green, as there may also be yellow coloured compounds in the oil.The intensity of the colour is believed to be dependent upon the amountof potentially sediment-forming compounds in the oil. The colour may beobserved by visually comparing the colour with a standard colour, forexample by using a reference sample or colour chart, or by means of acolorimetric instrument. However the colour is observed, it may berelated to the amount of these compounds in the oil, to the storage lifeof the oil or to its acceptability/non-acceptability for use, as isunderstood in the art. This may be done for example by initially testingan oil by both present methods and the method of the invention andpreparing a calibration chart for subsequent use of the method of theinvention.

The catalyst is any substance which has the properties of aLowry-Bronsted acid or by reaction with the oil or solvent can form aLowry-Bronsted acid, or can react directly with the nitrogen atom of anindole to form a salt.

Preferred acids are those which are colourless, and are known to formsalts with the indolyl phenalenone condensation product. For examplemineral acids such as hydrochloric or sulphuric and/or perchloric acidmay be used and give a rapid colour formation. Preferred acids areorganic sulphonic acids, such as alkyl or especially aryl sulphonicacids; for example, alkylphenyl sulphonic acids such as p-toluenesulphonic acid and napthalene sulphonic acid. Carboxylic acids such asacetic acid may be used but may give a slower reaction, a less intensecolour and a less sensitive test. Preferred acid derivatives arearomatic or aliphatic acyl chlorides or acid anhydrides, such as benzoylor acetyl chloride.

A rapid qualitative indication of the degree of stability of a fuel maybe obtained by a visual inspection of the type and intensity of colourproduced. A higher degree of quantification of the relative instabilityis obtained by dissolving the coloured species in a solvent andmeasuring the intensity and wavelength of the light absorbing speciesusing a colorimeter or spectrophotometer.

Therefore, in one embodiment of this invention, the coloured reactionproduct is caused to form a solution in a solvent and then the visiblecolour and/or colorimetric absorbance of said solution is related to thepresence and/or amount of the unstable compounds in the oil.

This solvent is preferably at least partly organic, wholly or partlyimmiscible with the oil, and consequently the colour and/or absorbancemay be observed in the separate solution phase. It is also preferred inthis embodiment that the acid catalyst is soluble in the solvent.Preferably the solvent is a polar solvent substantially immiscible withthe oil, and is a water-miscible organic solvent or an organicsolvent-water mixture. Preferred solvents are therefore alcohols,especially lower alcohols (i.e. up to C₅ alcohols), especially methanolor ethanol, or mixtures of these with water. The use of a solvent,particularly methanol, conveniently allows the acid catalyst to be addedto the oil in the form of a solution in the solvent. The acidcatalyst/polar solvent is preferably added to the oil at a fuel/solventratio from 20:1 to 1:4 volume/volume. The intensity of the colour whichdevelops with the unstable oil is then determined in the solvent layer,by visual observation or by absorbance measurement. In this embodimentthe method may be carried out at ambient temperature but is preferablycarried out at an elevated temperature, e.g. 20°-50° C.

The appearance of a blue, green or blue-green colour in the solventvisible to the naked eye, or which may be detected by a colorimetricinstrument at a wavelength between 600-850 nm, is associated withpotential instability of the oil. The intensity of the colour, and/orwavelength of the colorimetric absorption maximum may be related to theamount of sediment which is expected to form, and hence to a storagelifetime or acceptability relative to a quality control standard.

In an alternative form of this embodiment a solid acid catalyst iscontacted with the oil, a coloured reaction product is allowed to formon the surface of the catalyst, the oil and catalyst are separated, e.g.by decantation, and the solution is formed, e.g. by addition of thesolvent to the solid catalyst. Preferred solid catalysts arepara-toluene sulphonic acid or naphthalene sulphonic acid, and apreferred solvent is methanol.

In a second embodiment of this method an oxidising agent is alsocontacted with the oil, together with the acid. The use of an additionaloxidising agent is believed to assist in causing the oxidation ofphenalenes to phenalenone and phenalanones. In this embodiment it isalso preferred that the coloured reaction product is caused to form asolution as described in the first embodiment above and therefore thepreferred features of this first embodiment are included in this secondembodiment.

Any oxidising agent which is known to oxidise phenalenes to phenalenonesmay be used. Inorganic oxidising agents are preferred, particularlytransition metals in an oxidising oxidation state such as Ce IV, Mn VII(e.g. permanganate) or Cr VI (e.g. dichromate). These latter two give arapid reaction without interference from the colour of the reduced ion.Other suitable oxidising agents include I₂, IO₃ ⁻, BrO₃ ⁻ and C₁₀ ⁻. H₂O₂ may also be used but can give a slower reaction.

It is particularly preferred to carry out this second embodiment of themethod by contacting the oil with an oxidising agent, an acid, and asolvent which is at least partly organic and which is wholly orpartially immiscible with the oil, said oxidising agent and acid beingat least partly soluble in the solvent, then observing the colour of thesolvent phase, either visually or by measuring the absorbance.

Some acids are also oxidising agents, such as perchloric and nitricacids. Such oxidising acids may therefore in some cases be used in placeof both the oxidising agent and the acid, or may be used in addition toeither or both. Perchloric and nitric acids may for example be used asboth an acid and oxidising agent or may be used together with anadditional oxidising agent such as dichromate.

It is preferred to provide the oxidising agent as a solution, especiallywhen the acid catalyst is also provided as a solution. These solutionsmay both be made up in the same selected solvent, or if the solvent is awater-miscible organic liquid or an organic liquid-water mixture one maybe made up in this and the other in water. In this way all the materialsused in the test method may then conveniently be added as these twosolutions.

When an oxidising agent is used, a preferred solvent is a methanol- orethanol- water mixture solvent, preferably containing 70-80% by volumeof the alcohol. This mixture dissolves around 2-3 weight % of potassiumdichromate or permanganate. When the acid is provided in the form of asolution then this preferably contains 10% or less by weight of theacid. When the oxidising agent is a compound of a transition metal andis provided as a solution then this is preferably a saturated solution,especially when in water so that the minimum volume of water need beused in the method.

In use in a preferred form of this second embodiment, a sample of theoil is taken and to it is added the oxidising agent, acid and solvent.The mixture is then preferably agitated so as to bring all theingredients into intimate physical contact. The mixture is thenpreferably allowed to separate into the solvent phase and the oil phase,and then the colour of the solvent phase is observed. When the oxidisingagent and acid are added as one or more solutions, it or these may beadded to the sample of the oil, forming the solvent phase whichseparates from the oil.

When an oxidising agent is used as described above, the test method maybe used with small samples of the oil, at ambient temperature. The testtakes overall around 5-10 minutes, most of which time is spent waitingfor the solvent and oil phases to separate, and can detect as little as1-2 ppm of phenalenes or phenalenones. The quantities of the oxidisingagent and acid do not appear to be critical but very dilute reagents canlead to a slow production of a result. For consistency, it is importantto use as far as possible the same conditions for each test.Conveniently a sample of about 5 ml of the oil may be used, to whichneed be added no more than about 0.5-2 ml of solvent. When the acid andoxidising agents are in separate solutions, e.g. the preferred solutionsdescribed above, then typically 0.5 ml of the acid solution and 1 dropto 0.5 ml of the oxidising agent need be used.

In the above two embodiments, the rate and intensity of colourdevelopment may be enhanced by carrying out the method in the presenceof a solid material, preferably a white material. Preferred solidmaterials are silica, e.g. chromotographic silica, silicaceous earthssuch as Kieselgel G, and calcium sulphate.

The solid material may be simply added to the mixture of oil, acid, andif present, solvent and oxidising agent, or in a preferred form it maybe mixed with the acid catalyst prior to addition of the oil. In such amixture the catalyst is preferably present in concentrations of 0.1% to10% by weight. Preferred mixtures are of chromotographic silica,silicaceous earth or calcium sulphate with solid para-toluene sulphonicacid. Alternatively the solid material/catalyst mixture may comprise asilica substrate having a sulphonic acid chemically bonded thereto, forexample as commercially available strong cation exchange media whereacids such as benzene sulphonic and propyl sulphonic are chemicallybonded to silica. One such material containing benzene sulphonic acid issold under the trade mark BOND-ELUT SCX.

The solid material may physically support the acid catalyst. It may actas a background to the colour or it may absorb the coloured product. Inthis latter situation the support may be allowed to settle, then beseparated from the oil, and the coloured product washed from the supportinto solution with a solvent.

Preferably the oil:solid material ratio is in the range of 1:1 to 500:1by volume if it is added into the mixture as described above.

In the above descriptions the mixture of materials with the oil and eachother may be carried out in any order or simultaneously.

In a third embodiment of this method the solid material/acid catalystmixture is provided in the form of a column through which the oil isallowed to pass, or in the form of a layer on a solid backing alongwhich the oil is allowed to flow. The flow may be in a manner analogousto a TLC plate or the layer and backing may be dipped into or otherwisecoated with the oil or a solution of the oil and allowed to drain.Preferred acid catalysts, solid supports and mixtures thereof are asdiscussed above.

Short columns may be used, e.g. 10 mm long by 5 mm diameter, requiringonly 5 ml of the oil. In such a column the support material ispreferably in the form of a powder, especially 100-200 mesh size. Inboth columns and layers the concentration of the acid catalyst in themixture may vary along the length of the column or across a dimension,e.g. length, of the layer.

When such columns and layers are used, the coloured product imparts avisible blue or blue-green colour to the column or layer which may berelated to the presence and/or amount of unstable compounds in the oil.Alternatively the coloured product may be caused to form a solution in asolvent such as methanol by elution from the column with the solvent,the visible colour or colorimetric absorbance of which may be observedand related to the presence and/or amount of unstable compounds in theoil.

In this third embodiment the intensity of the colouration on the columnor layer may be enhanced by adding an oxidising agent to the oil priorto passing it through the column or contacting the layer with it.Preferred oxidising agents are as discussed above with reference to thesecond embodiment, but hydrogen peroxide is particularly preferred. Theoxidising agent may be added to the oil neat or in solution in asolvent.

A fourth embodiment is to pass the oil through a column of solid supportmaterial which retains the unstable compounds from the oil.Chromatographic silica, 100-200 mesh size, is a preferred supportmaterial. The unstable compounds are then removed from the column bywashing with a polar solvent, methanol being preferred. The addition ofan acid catalyst to the methanol washings then imparts a blue orblue-green colour, the visible colour or colorimetric absorbance ofwhich may be observed and related to the presence and/or amount ofunstable compounds in the oil. Similar columns and similar amounts ofmaterials to those described in relation to the third embodiment may beused.

For convenience test kits may be provided containing in combination thematerials and reagents necessary for performing the test method of thisinvention. For example a test kit for performing the first embodimentmay include a solution of an acid catalyst in a suitable solvent. Forperforming the second embodiment the test kit may include separatesolutions of an acid catalyst and an oxidising agent in suitablesolvents. The kits for the first and second embodiments may also includea powdered solid support material which may be prior mixed with the acidcatalyst, and solvent. For performing the third embodiment the kit mayinclude a ready made column, or layer on a backing, of a support/acidmixture, a solvent, and an oxidising agent. The kits may also includesuitable vessels for performing the test, and/or colour comparisoncharts.

Non-limiting examples illustrating the invention follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows colour development with time on a column.

FIG. 2 shows the spectrum of coloured reaction product in methanol.

FIG. 3 shows the relationship between colorimetric absorbance and lightcycle oil content in three fuel oil samples.

The following reference fuels, on which comparative studies have beencarried out, are referred to:

Fuel A: Straight run automotive distillate from Refinery 1.

Fuel B: Straight run automotive distillate from Refinery 2.

HCCLCO: Hydrogenated catalytically cracked light cycle oil.

TCLCO: Thermally cracked light cycle oil.

CCLCO: Catalytically cracked light cycle oil.

Fuel C: Blend of 30% HCCLCO with Fuel B.

Fuel D: Blend of 5% TCLCO with Fuel A.

Fuel E: Blend of 5% CCLCO with Fuel B.

Fuel F: Blend of 30% TCLCO with Fuel A.

Fuel G: Blend of 30% CCLCO with Fuel B.

The relative stability of each of the reference fuels was determinedusing the method of DEF STAN 05-50 (Part 40/1--long method). The amountof insoluble matter formed on ageing is shown in the Table.

    ______________________________________                                                   TOTAL INSOLUBLE                                                    FUEL       mg/1                                                               ______________________________________                                        Fuel A     5                                                                  Fuel B     4                                                                  Fuel C     3                                                                  Fuel D     13                                                                 Fuel E     9                                                                  Fuel F     54                                                                 Fuel G     43                                                                 ______________________________________                                    

Fuels D, E, F and G are considered unstable.

EXAMPLE 1

Ten drops of one of the following liquid catalysts or 20 mg of one ofthe solid catalysts: concentrated hydrochloric acid, concentratedsulphuric acid, benzoyl chloride, para-toluene sulphonic acid,naphthalene sulphonic acid, dodecylbenzene sulphonic acid; weredissolved in 20 ml of methanol and added to 50 ml of each referencefuel. The methanol is not fully miscible with the fuels and a two phasemixture was formed. The mixture was vigourously shaken after mixing thecatalyst and again after 23 hours at 43° C. After standing for a furtherhour at 43° C., no blue colouration was apparent in the methanol layerwith the stable Fuels A-C, a pale blue colouration was apparent withunstable Fuels D and E and a darker blue colouration with F and G.

Detection of the blue colouration was quantified by measurement of thelight absorbance of the methanol layer in the region 600 to 850nanometers using a ultraviolet/visible spectrophotometer. The absorbanceof the methanol layers from the stable Fuels A-C at 770 nanometers in a1 cm pathlength cell was less than 0.01. The absorbance readings for theunstable fuels at this wavelength were as follows: D, 0.37; E, 0.23; F,1.76; G, 1.48.

The intensity of the blue colour increased with increasing temperatureof the fuel/methanol/catalyst mixture and with time beyond the 24 hours.

EXAMPLE 2

Solid crystals of para-toluene sulphonic acid or o naphthalene sulphonicacid were added to fuel. A dark blue colouration was produced onundissolved solid after 24 hours standing at 43° C. with the unstablefuels. This blue colouration was readily seen by decanting the fuel andadding 5 ml of methanol to the solid residue. No blue colouration wasobserved in the methanol solution with the stable fuels, a mediumcolouration was obtained with unstable Fuels D and E and a dark bluecolouration with unstable Fuels F and G.

EXAMPLE 3

Oxidising Agent: Saturated solution of potassium dichromate or potassiumpermanganate in water or of potassium dichromate in aqueous methanol(70-80% v:v methanol).

Acid/Organic Solvent: Solution of p-toluene sulphonic acid in methanol(10 wt %).

Approximately 5 ml of a diesel fuel were placed in a 10 ml graduatedtest tube. 0.5 ml of the acid solution was added followed by one drop ofthe oxidising agent solution. The tube was stoppered and the mixture wasvigorously shaken for one minute then allowed to stand until twodistinct layers had formed, the methanolic layer being uppermost. Thistook 5-10 minutes. The colour of the methanolic layer was observed andrecorded by a colour photograph.

This method was applied to 5 samples of diesel oil of decreasing longterm storage stability in respect to sediment formation. These were (1)straight run ("SR") gas oil (known to be stable); (2), (3), (4) and (5)respectively 0.1%, 1%, 5% and 10% catalytically cracked blend in SR gasoil. Samples (4) and (5) produced a deep green colouration of themethanolic phase which was found to represent an unacceptable level ofsediment formation on storage. With samples (1) and (2) no greencolouration at all was visible to the naked eye, and in sample (3) agreen ring was visible at the junction between the oil and solventlayers but no green colouration was visible to the naked eye in thesolvent phase itself. Samples (1) and (2) in which no hint of greencolouration was visible to the naked eye, were also tested by presentstandard tests for diesel oil and found to have an acceptable level ofstorage stability to a UK Ministry of Defence Standard requiring longterm formation of no more than 2 mg of sediment per 100 g of fuel onstorage whilst samples (3), (4) and (5) when tested by the presentstandard test were found to yield an unacceptable amount of sediment bythis MOD standard.

EXAMPLE 4

Solid para-toluenesulphonic acid (0.1 g) was ground with 100-200 meshchromatographic silica (10 g) and the powder added to a 5 mm diameterglass or plastic tube to a height of 10 mm above a plug of glass wool toretain the powder. Each fuel (1.0 ml) was added to a separate tube abovethe silica. The fuel was allowed to percolate through the powder undergravity. With unstable Fuels F and G, medium green/blue colourationswere produced in the silica within one minute of addition of the fuel.Within 5 minutes no fuel remained above the silica and the mediumgreen/blue colouration was spread throughout the silica. A lightergreen/blue colouration was similarly produced with unstable Fuels D andE, while no colouration was produced with the stable Fuels A, B and C.

EXAMPLE 5

Separate tubes containing silica were similarly prepared (10 g of silicaground with 0.1 g of acid catalyst) with each of the following acidiccatalysts: naphthalene sulphonic acid, dodecylbenzene sulphonic acid,concentrated hydrochloric acid (37%), concentrated sulphuric acid (98%),benzoyl chloride, and acetyl chloride. Addition of unstable Fuels F andC (1 ml) to all tubes produced in each case a green/blue colouration inthe silica within 1 minute of addition of the fuel. Within 5 minutes thefuel had percolated through the silica and green/blue colour was evidentthroughout the column. With the unstable Fuels D and E the colour wasless intense, while the green/blue colour was absent with each of thestable fuels, A, B and C.

EXAMPLE 6

The TCLCO and CCLCO, blended to produce the unstable fuels used in thetests described above, were frozen at -15° C. within seven days of beingobtained from the refinery. A further series of unstable fuels wasprepared with the same proportion of TCLCO and CCLCO, but using TCLCOand CCLCO which had been standing at ambient temperatures for threemonths. Blue/green colour development was observed within 5 minutes onthe treated silica tubes for those fuels containing the freeze-storedLCO. However, with the three ambient aged LCO blends a dark blue bandwas formed at the top of the silica column and there was less intenseblue/green colouration throughout the silica. The intensity of the darkblue band at the top of the silica column was an indication of the ageof the LCO in the unstable fuel blend.

EXAMPLE 7

Variations of the method of addition of the acidic catalysts to thesolid silica were to dissolve the material in a solvent. For example,para-toluene sulphonic acid (0.1 g) was dissolved in methanol (10 ml)and a slurry formed with the silica. The methanol solvent was removed byevaporation with gentle heating and the column prepared from the driedsilica as previously. Alternatively, the column could be packed withuntreated silica and methanol/acid catalyst (0.5 ml) added above thesilica. Gentle heating of the column was then sufficient to evaporatethe methanol. In all cases with columns prepared with these variants,blue/green colouration was produced with the unstable Fuels D-G and notwith the stable Fuels A-C. The intensity of the colouration wasproportional to the amount of acid remaining on the column. This couldbe varied by the initial concentration in the solvent, by the amount ofsolvent added to the column and by the number of solvent additions.

EXAMPLE 8

Columns were also prepared in which the amount of acid catalyst variedalong the length of the column. For example, para-toluene sulphonic acid(0.1 g) was dissolved in a mixed isopropanol/toluene solvent (1:4 byvolume). The solvent/acid catalyst (1 ml) was added to the silica columnfollowed by 2 ml of hexane. After allowing the column to air dry, 1 mlof each of the test fuels was added to individual columns in a counterdirection to that of addition of the solvent/acid catalyst. With each ofthe unstable fuels, a graduation of blue/green colour was obtained fromthe top (where the fuel was added) to the bottom of the column, with thedarkest colouration being at the bottom corresponding to the highestcatalyst concentration. Variations of intensity of colour were readilyseen by comparison of the colour graduation for tubes from each of theunstable fuels D-G. The intensity of the colour was proportional to theamount of sediment produced in the fuel as listed in the Table above.

EXAMPLE 9

Acid catalyst was also deposited on a "thin layer" coating of silica ona flexible polymer backing plate from a methanol (5 ml)/acid catalyst(0.1 g) solution. In this case, the flexible polymer plate and silicacoating was immersed into the acidic solution and removed after all thesilica coating was fully wetted with solvent. The methanol solvent wasallowed to air evaporate to leave residual acid catalyst. The treated"thin layer" plate was immersed into the test fuel until the silicalayer was fully wetted with the fuel and then withdrawn. As previously,the intensity of the blue/green colouration produced on the silicacoating was proportional to the amount of sediment produced in theunstable fuels.

EXAMPLE 10

A variation of this coating procedure was to apply successive coatingsof residual acid catalyst to portions of the plate. As an example, thecomplete silica coating of the plate was wetted with the acidcatalyst/methanol (ACM) solution, and the methanol solvent allowed toevaporate. Then 80% of the silica coating was wetted with a second AMCsolution followed by air evaporation of the solvent. This was similarlyfollowed by 60% (three ACM wettings), 40% (four ACM wettings) and 20%(five ACM wettings) of the silica surface being wetted with acidcatalyst/methanol solvent. The five concentration variations of acidcatalyst were obtained along the silica plate corresponding to thenumber of wettings with ACM solution. When this plate was immersed intothe unstable fuels and withdrawn after being wetted with fuel, ablue/green colouration was again obtained, which was proportional to theamount of acid catalyst deposition on the silica and the amount ofdeposit produced in the fuel.

EXAMPLE 11

In addition to the columns produced with silica, Kieselgel G, andanhydrous calcium sulphate were used as solid supports for the acidcatalysts applied as described above. Blue/green colourations were againobtained on the solids with the unstable fuels, although the relativeintensities of the colours were not as great as with the silica supportand the relative distribution through the solids was not as even.

EXAMPLE 12

Two Fuel D samples were added to a silica/para-toluene sulphonic acidcatalyst column. The first such sample was Fuel D prepared as above. Inthe second sample, one drop of 100 volume hydrogen peroxide was added to1 ml of Fuel D and the mixture was allowed to stand at ambienttemperature for 60 minutes before addition to the column. The intensityof the blue/green colouration in the column was greater with the samplecontaining hydrogen peroxide. Other oxidising agents similarly producedan enhancement of the colouration.

EXAMPLE 13

The colour produced in the solid columns was quantified by passingmethanol (2 ml) after all the fuel had passed through the column andmeasurement of the intensity of the colour extracted into the methanolin the range 600 to 850 nanometers. For example,following washing withmethanol (2 ml) 60 minutes after passage of unstable fuels D and F (1ml) through silica/para-toluene sulphonic acid catalyst column,absorbances at 770 nanometers of 0.11 and 0.29 respectively wereobtained in a 10 mm pathlength cell.

EXAMPLE 14

A 4 cm long Column of the silica-p-toluene sulphonic acid mixture usedin Example 4 was made in a 5 mm diameter clear glass tube. A syringe wasused to add 2 ml of fuel oil to the tube. Gentle pressure was applied tocause the oil to flow through in 30 seconds. Colour development occurredimmediately and in most cases was complete within 10 minutes.

Spectral measurements were obtained after washing the column with 1 mlof hexane to remove residual oil, followed by 6 ml of methanol. In eachcase gentle pressure was applied to cause the liquid to flow through in30 seconds Spectra of the methanol wash liquid were recorded immediatelyusing 10 mm pathlength cells.

FIG. 1 shows the rapidity of colour development. Columns were washedafter period of 2-1500 minutes following addition of 2 ml of fuel oilcontaining 15% of light cycle oil (LCO) obtained directly fromAustralian refineries within one week of production. From 300 to 1400minutes at ambient temperature the absorbance of 650 nm was relativelyconstant. FIG. 2 shows the spectrum of column methanol washings after300 minutes.

Results from 5 blends of LCO from 3 refineries are shown in FIG. 3. Thevariation of absorbance with LCO concentration was approximately linearfrom 0 to 30% LCO content.

EXAMPLE 15

A 4 cm long column of chromatographic silica (100-200 mesh) was preparedin a 5 mm diameter glass tube. A plastic tube was used to add 2 ml offuel oil to the tube. Gentle pressure was applied to cause the oil toflow through in 30 seconds.

The column was washed with 1 ml of hexane followed by 6 ml of methanol.Two ml of a 10% solution of p-toluene sulphonic acid in methanol wasadded to the methanol washings and the resultant colour noted and itsabsorbance in the 600-850 nm region measured between one and 16 hoursafter the acid addition.

Comparison of results from ASTM D4625 and oxygen overpressure tests withthe method of the invention showed that fuels producing the mostsediment also produced the highest absorbance.

LCO has a high aromatic content and also is a better solvent thanstraight run distillate (SRD) for potential sediment forming materials,which can consequently form injector and cylinder deposits. The testmethod of the invention easily allows the amount of LCO in a fuel oil tobe determined.

We claim:
 1. A method of testing a fossil-fuel derived hydrocarbon oilcontaining an indole for the presence of chemically unstable compoundsselected from the group consisting of phenalenes, phenalanones andphenalenones, wherein a sample of the oil is contracted with aLowry-Bronsted acid catalyst to form a colored reaction product betweenthe indole and the unstable compound when present in the sample; and thecolor of the sample within the region between 600-850 nm is visuallyobserved and the presence of at least one of said unstable compounds insaid sample is determined by comparison to a predetermined standard. 2.A method according to claim 1, wherein the acid catalyst is selectedfrom the group consisting of: mineral acids, carboxylic acids, aryl oralkyl sulphonic acids, aryl or alkyl acyl chlorides, and aryl or alkylacid anhydrides.
 3. A method according to claim 1 wherein the coloredreaction product is brought into solution in a solvent which is at leastpartly organic and is at least partly immiscible with the oil prior torelating the visible color of the reaction product to the amount ofnamed unstable compound in the oil.
 4. A method according to claim 3wherein the intensity of visible color is observed in the solution phaseand the acid catalyst is soluble in the solvent.
 5. A method accordingto claim 4 wherein the solvent is selected from the group consisting ofC₁ to C₅ alcohols and mixtures of such alcohols with water.
 6. A methodaccording to claim 4 wherein a solution of the acid catalyst in thesolvent is contacted with the oil.
 7. A method according to claim 4wherein: the acid catalyst is a solid; colored reaction product forms onthe surface of the catalyst; the oil and catalyst are separated; and,the solution is thereafter formed.
 8. A method according to claim 1wherein an oxidizing agent capable of converting a phenalene to aphenalenone is contacted with the sample.
 9. A method according to claim8 wherein to the oil are added: the oxidizing agent; acid catalyst; and,a solvent selected from the group consisting of C₁ to C₅ alcohols andmixtures of such alcohols with water, and which is wholly or partiallyimmiscible with the oil; said oxidizing agent and acid catalyst being atleast partly soluble in the solvent: and, wherein the color of thesolvent phase is then visually observed.
 10. A method according to claim9 wherein a solution of the oxidizing agent in the solvent is contactedwith the sample.
 11. A method according to claim 10 wherein a solutionof the acid in the solvent is contacted with the sample.
 12. A methodaccording to claim 1 wherein: the sample; acid catalyst; and an optionalsolvent selected from the group consisting of C₁ to C₅ alcohols andmixtures of such alcohols with water; and an oxidizing agent capable ofconverting a phenalene to a phenalenone, are contacted in the presenceof a solid support material.
 13. A method according to claim 12 whereinthe solid support material is mixed with the acid catalyst prior tocontact with the sample.
 14. A method according to claim 13 wherein themixture of solid support material and acid catalyst contains 0.1 to 10weight % of the acid catalyst.
 15. A method according to claim 13wherein the acid catalyst plus support material mixture is provided inthe form of a column through which the oil sample is passed.
 16. Amethod according to claim 15 wherein the colored reaction product iseluted from the column with a solvent to form a solution.
 17. A methodaccording to claim 15 wherein the concentration of acid catalyst in theacid catalyst plus support mixture varies along the length of thecolumn.
 18. A method according to claim 15 wherein the oil is treatedwith oxidizing agent prior to causing it to flow through the column. 19.A method according to claim 13 wherein the acid catalyst plus supportmixture is provided in the form of a layer on a solid backing, overwhich layer said sample is allowed to flow.
 20. A method according toclaim 19 wherein the concentration of acid catalyst in the acid catalystplus support mixture varies over a dimension of the layer.
 21. A methodaccording to claim 19 wherein the oil is treated with oxidizing agentprior to causing it to flow over the layer.
 22. A method according toclaim 1 wherein reactive compound is separated from the oil beforeunstable compound is contacted with the acid catalyst.
 23. A test kitfor performing a method according to claim 1 and including aLowry-Bronsted acid catalyst supported by a solid material, an oxidizingagent capable of converting a phenalene to a phenalenone, and a colorcomparison chart having a range of color absorption intensities in theregion 600-850 nm.
 24. A test kit according to claim 23 wherein thecatalyst and supporting solid material is in the form of an enclosedcolumn.
 25. A test kit according to claim 23 wherein the catalyst andsupporting solid material is laid on a solid backing.
 26. A test kit forperforming a method according to claim 1 and including a Lowry-Bronstedacid catalyst, a solvent selected from the group consisting of C₁ to C₅alcohols and mixtures of such alcohols with water, an oxidizing agentcapable of converting a phenalene to a phenalenone, and a chart enablingcomparison of spectrometric test results with spectrometric results onpredetermined standard materials in the region 600-850 nm.
 27. A methodaccording to claim 9, wherein intensity of color of the solvent phase isvisually estimated against a chart standard.
 28. A method according toclaim 9, wherein intensity of color of the solvent phase is measured inthe visible spectrum in the range 600-850 nm.
 29. A method according toclaim 1, further including a step of measuring intensity of color of thesolvent phase.
 30. A method according to claim 29, wherein intensity ofcolor of the solvent phase is measured by using a colorimeter orspectrophotometer.